1. Field of the Invention
Documents such as identity cards, admission cards, credit cards, papers representing value, checks, travel tickets and the like, with machine-readable information, are known in many forms. Most documents presently used have coded information in the form of magnetic or optical markings, and can be forged at relatively low cost.
2. Description of the Prior Art
A very high degree of non-forgeability is achieved in a known machine-readable document which includes a hologram, which has the holographic image of a characteristic number which is coded in binary form by a given pattern of mutually demarcated spots of light. A hologram of this kind, which includes a coded characteristic number of authenticity data, can be relatively easily read and checked for authenticity by machine. On the other hand, the production of such a hologram requires expensive technical aids which are difficult to obtain, and well-founded technical knowledge, so that successful forgeries are only possible at extremely high cost. In a document with such a hologram, the characteristic number is produced thereon in the process of forming the holographic image of the pattern of spots of light--that is to say, at an early stage in the production process. This means that, for each document with which an individual characteristic number is to be associated, a respective particular hologram must be applied, and the individual characteristic numbers of a series of documents must be known to the producer of the holograms.
It has also already been proposed that a multiplicity of machine-readable optical markings which produce a characteristic modification of incident light by diffraction or refraction may be applied to a document, and selected markings may subsequently be cancelled, so that the geometric position of the markings remaining on the document represents coded information. This makes it possible for the coded information to be applied to the document in a very simple manner, while nonetheless retaining the advantages of storing the authenticity data in the form of light-modified markings. The authenticity of the markings can be checked and the coded information read with an optical reading device.